The invention relates to apparatus for positioning a prosthetic acetabular cup within an acetabulum during total hip replacement surgery.
As part of total hip replacement surgery, a prosthetic acetabular cup is implanted within the human acetabulum to substitute for the socket of the human hip joint. This is usually done to compensate for severe damage of the acetabulum due to disease, trauma, or other factors. A prosthetic femoral component is mated with the acetabular cup component to complete the total hip replacement surgery. In order to achieve optimal performance of the combined acetabular and femoral prostheses, the acetabular cup must be properly positioned in the acetabulum. An improperly positioned acetabular component can lead to dislocations of the hip joint, decreased range of motion, and eventual loosening or failure of both the acetabular and femoral components.
Interoperative placement of the acetabular component can be a surgically demanding task. The orthopedic surgeon must adequately fix the cup in the proper alignment while hampered by limited surgical exposure and relatively few clear reference points. A typical means of fixation is a biocompatible grouting agent such as polymethyl methacrylate bone cement. In order to achieve adequate performance from such a cement, it is necessary to ensure that complete filling is obtained, with minimal voids, air bubbles, etc. This requirement presents another interoperative difficulty for the surgeon; i.e., adequate pressure must be maintained on the prosthetic component during the early stages of cement polymerization while avoiding relative motion between the cup component and the cement. Any rocking or twisting of the cup in the cement can compromise good cement filling, by pulling the soft cement away from the bone or away from the surface of the cup.
Traditional cup positioners generally include at least a pusher ball sized and shaped to fit the recess within the acetabular component, a positioner arm, a positioner flange juxtaposed the pusher ball, and a pusher arm connected to the ball and to the flange to enable the user to push the prosthetic acetabular cup, when resting on the flange, into the prepared acetabulum. Generally, the cup must be manually held against the flange during implantation. In some cases, the cup is aligned on the flange by means of small projecting pins which can engage complementary holes in the face of the cup. Such pins require the surgeon to always select the cup and the cup positioner from the same manufacturer, to ensure that pins and holes will mate properly. While such pins can help to orient the cup relative to the positioner, it is not believed that they prevent the cup from falling off the flange, particularly in the case of heavier metal-backed acetabular components; the cup must still be manually held against the flange as described above. In the case of U.S. Pat. No. 4,475,549, a pusher ball is described at column 2, lines 33-35 as provided with a cover 23 of a compressible material, such as silicone, which has a relatively high coefficient of friction. In order to remove the cup from this frictional fit, a rigid blunt rod, described at column 4, lines 12-15, is required to press against the cup. This change of instruments can, however, disturb the cup in the cement and compromise good cement filling.
Once the cup has been aligned and fully implanted or inserted, the flange has served its purpose and, if left in contact with the cup, can cause the cup to be moved as described above. There is also a risk of the cement bonding to the flange. Further, removing any excess cement from around the periphery of the cup is more difficult when the flange is obscuring the view of the cup. Separate cup pushers are often used in conjunction with a cup positioner to address some of these problems. The positioner is removed once the cup has been placed in the acetabulum, and the pusher is separately used to apply pressure to the cup. However, the possibility exists that this change of instruments will dislodge the cup or rock it in the soft cement. U.S. Pat. No. 4,305,394 addresses this possibility and describes a means for disengaging the flange so that the flange no longer contacts the rim of the cup. The flange is disengaged by means of a trigger mechanism during cement polymerization, but the flange still remains within the surgical field and can obscure the view of the cup.
In addition to the pusher ball, the flange, and the pusher arm, some acetabular cup positioning instruments include means for establishing reference angles to better insure proper alignment of the prosthetic cup. Many traditional positioner designs include a reference arm for establishing the abduction angle. U.S. Pat. No. 4,475,549 further includes means for establishing the anteversion angle. In the case of U.S. Pat. No. 4,475,549, the anteversion angle is established relative to the positioner arm, as described starting in column 3, line 12. This is, it is believed, difficult to accomplish since the line of sight of the physician user is not parallel with or in the same plane as the plane defined by the arms 33 and 35, as best shown in FIG. 5; there can be a tendency to rotate the positioner 11 about the vertical axis shown in FIG. 5 until the handle 51 is aligned with the patient axis 53, thereby adding undesired cup rotation to the anteversion angle.